The most common meters for measurement of neutron dose (remmeters) are based on neutron moderators, with a thermal neutron detector in the center. Bramblett et al (1960) initiated this design when they proposed that a 12-inch diameter polyethylene sphere having a 6LiI(Eu) thermal neutron detector at its center provides a reasonable approximation to the dose-equivalent curve for neutrons (ICRP (1969)) from thermal to approximately 20 MeV. The dose-equivalent response of such a detector, calculated using GEANT4, a modern Monte Carlo simulation code (Agostinelli et al (2003)), shows that such a remmeter still over-responds by over a factor of 3 at around 10−3 MeV and increasingly under-responds above 10 MeV.
Over the years, many variants of this moderator design were produced by other groups (Nachtigall (1962), Leake (1966); Hankins (1967)). Such remmeters are currently commercially available from several vendors (Canberra, Ludlum, Berthold, Tech-Associates, Owen Scientific), with the 6LiI(Eu) detectors often replaced by small 3He gas counters to improve gamma discrimination. Other moderator designs have replaced the spherical moderator by a cylindrical moderator (Anderson and Braun (1964); Widell and Svansson (1973); Hankins (1978)), which sacrifices isotropic detection performance. The major problem with the use of a larger moderator is that the remmeter is too heavy (>15 kg) for ease of use in many operational scenarios.
There has always been a desire for a lighter remmeter. Manufacturers have produced remmeters using smaller moderators, but the physics of neutron moderation is such that dose-equivalent accuracy is sacrificed. For example, when one considers the dose-equivalent response of an 8″-diameter and a 5″-diameter polyethylene sphere, it can be seen that the over-response of the 8″ sphere at 1 keV is about 15 and over 50 for the 5″ sphere, with correspondingly poorer performances above 10 MeV. It is generally recognized that moderators smaller than about 8″ would be too inaccurate for practical neutron dosimetry over a broad energy range. Unfortunately, a remmeter based on even an 8″ moderator is still too heavy (>5 kg) for convenient operational use.
Our company produced a light (4.1 kg) neutron remmeter over 10 years ago (Ing et al (2007)), based on spectral dosimetry. Our Microspec Spectroscopic Neutron Probe (MSNP) was intended for extremely accurate neutron dosimetry for use by specialists, but not for direct competition with conventional remmeters, because it was not sufficiently robust for many field uses. This probe uses a hydrogenous liquid scintillator with neutron/gamma discrimination capability (commonly used for neutron spectroscopy in laboratories) as the detector for fast (>0.8 MeV) neutrons and a 3He gas counter, embedded in a thick 10B shell of special design, as the detector for thermal and intermediate energy (<0.8 MeV) neutrons. The 10B shell was designed so that the 3He counter would have a dose-equivalent response that closely mimics the dose-equivalent curve over this energy region. Tests done by ourselves and others (Devine et al (2002)) confirm the high dosimetric accuracy of this product.
There have been developments of light remmeters by other groups (Olsher et al, (2004), Mourges et al (1984)) and products by various manufacturers (Canberra, Ludlum, Health Physics Instruments). However, these are for more restricted or specialized radiation fields and do not perform well as general purpose remmeters for a variety of operational neutron fields.
The recent development of a plastic scintillator (Zaitseva et al (2012)) that has neutron/gamma discrimination properties comparable to that of the traditional hydrogenous liquid scintillator (BC 501A) to permit the electronic separation of neutron and gamma-ray signals, provides a basic technology for the development of a general purpose light remmeter along the lines of our MSNP. The new plastic scintillator overcomes the major operational weaknesses of the previous liquid scintillator by allowing its use below freezing temperatures and eliminating the issue of failure of the liquid seal. The use of the new scintillator enables the development of the long-sought light-weight remmeter.
However, the use of the new plastic scintillator only enables the measurement and dosimetry of fast neutrons of ˜0.8 MeV and above. In the MSNP, the thick special 10B shell with the embedded 3He counter performs the dosimetry of thermal and intermediate energy neutrons (˜0.025 eV to 0.8 MeV). While this same approach could be used with the new plastic scintillator, it is not ideal because of the high costs of both 10B and 3He and the limited global supply of the latter.
According to one embodiment of the invention, there is provided an alternative and technically-superior approach to the dosimetry of neutrons in the thermal to intermediate energy region. It uses the properties of two different size moderators (whose combined weight is much less than a single large moderator) to determine the energy (or distribution of energies) of the neutrons in this energy region.